In one type of conventional power line communication systems, communication terminals that perform communication with one another by superimposing signals onto the power lines are distributed throughout a building. Examples of such communication terminals include various types of home electronic appliances in the case of general households, or lighting fixtures and personal computers in the case of office buildings. The communication terminals are connected to the power lines and act as load devices in the power line communication system.
These load devices often have an extremely low impedance compared to the characteristic impedance of the power lines in the signal frequency band used for the power line communication. As the number of load devices connected to the power lines increases, signal attenuation in the power line communication system is increased due to the impedances corresponding to the number of such load devices. This is one cause of an increase in the communication error rate. Furthermore, some load devices generate high-level noise, which is another cause of an increase in the communication error rate.
Several methods have been proposed to alleviate an increase in the communication error rate. Examples of such methods include attaching impedance boosters to problematic load devices, and relaying communication between terminals by using a relay device. One example of such methods is to place a dedicated relay device inside or in the vicinity of a distribution board, which is disclosed in Patent Literature 1.